This invention pertains to a catalytic two-stage hydroconversion process for coal or petroleum asphaltene feed materials to produce high yields of hydrocarbon distillate liquid products. It pertains particularly to such process which uses a large-pore catalyst in the first stage reactor and a small-pore catalyst in the second stage reactor, and to the catalysts used in the process.
The primary energy sources today or in the near future are coal and petroleum. In several regions of the United States, there are abundant supplies of various types of coal including bituminous, semibituminous and subbituminous, as well as lignite. To meet the ever increasing demand for transportation fuels, many methods have been disclosed in the prior art to convert coal into liquid fuels. Since the supply of petroleum is becoming depleted, the new upgrading technologies today are aimed at the conversion of petroleum asphaltene, which is the bottom of the barrel of crude oil, very heavy petroleum crudes or heavy hydrocarbon materials originated from tar sand or oil shale.
Coal and petroleum asphaltene are hydrocarbons having large molecular weights and very complicated molecular structures. For the production of distillate liquid fuels from coal, a step-wise series of reactions are envisioned: coal.fwdarw.preasphaltene (benzene-insoluble resid).fwdarw.asphaltene (benzene-soluble hexane-insoluble resid).fwdarw.distillate. For petroleum asphaltene processing, the reaction for producing distillate liquids proceeds likewise as follows: asphaltene (benzene-soluble hexane-insoluble).fwdarw.oil (hexane-soluble).fwdarw.distillate liquid.
Because of the ever increasing demand for transportation fuels relative to other energy needs, coal or petroleum asphaltene processing to yield a high percentage of distillate product is needed. Numerous methods have been disclosed in the art for these purposes. For coal processing, a single-stage catalytic process using an ebullating bed catalytic reactor is disclosed in U.S. Pat. No. 3,519,555; the catalyst is described as a hydrogenation catalyst selected from the group consisting of cobalt, molybdenum, nickel, iron and tin supported on a base selected from the group consisting of alumina, magnesia and silica. A multistage process for the production of hydrocarbons from coal employing a series of ebullating bed reactors is disclosed in U.S. Pat. No. 3,594,305. This process discloses that two or more of a first group of ebullating bed reactors effect removal of sulfur and oxygen and effect some hydrogenation, using as catalyst a supported sulfided Co-Mo, Ni-W, or Ni-Mo catalyst, with the temperature and pressure of reaction within the reactors being increased in each subsequent reactor as the product passes downstream, and a final group of reactors containing a noble metal catalyst at higher temperatures and pressures than in the first reactor series effect removal of nitrogen compounds and hydrogenate aromatic compounds.
The Solvent Refined Coal (SRC) Process, which recycles process hydrocarbon solvent to donate hydrogen for coal liquefaction, has high yield of unconverted coal plus resid. The Exxon Donor Solvent (EDS) Process is described by Ansell et al in the American Chemical Society, Division of Fuel Chemistry Preprints, Vol. 25, No. 3, 1980, p. 269. Using a hydrogenated recycle solvent, the process is still limited by low conversion of coal and resid. With vacuum bottoms material (1000.degree. F.+) recycling, the coal liquefaction and conversion to 1000.degree. F.- product is raised from 63 to 83 wt % of MAF Illinois No. 6 coal (Monterey mine) accompanied by an increase in hydrogen consumption from 4 to 6% wt. basis MAF coal. The high mineral-containing bottom fraction requires further processing for hydrogen production.
A two-stage coal liquefaction process called SRC I-LC Fining, is currently being developed. The first stage reaction is thermal, using recycle solvent from the catalytic second stage reaction to effect coal liquefaction. This two-stage process yields high percentage 1000.degree. F.+ materials like the single-stage donor solvent processes. However, at the same distillate (1000.degree. F.-) production level, the hydrogen consumption of this two-stage process is significantly lower than that of the EDS Process. The lower hydrogen consumption of this two-stage process is attributed to the lower operating temperature of the second stage reactor resulting in lower gas production.
The catalyst employed in coal liquefaction processes includes a variety of catalytically active materials on porous supports having large surface area. As stated in the background of U.S. Pat. No. 3,635,814 to Rieve et al, the desired pore size for a catalyst is about 50 to 250 .ANG. with the most frequent pore size being 60 .ANG..
The pore structure of petroleum resid hydrodesulfurization catalyst is disclosed in a number of patents. U.S. Pat. No. 3,509,044 favors the exclusion of asphaltene by maximizing surface area contained in pores having diameters of 30-70 .ANG.. U.S. Pat. No. 3,531,398 discloses an upper limit on the amount of macropore volume represented by pore diameters greater than 100 .ANG.. U.S. Pat. No. 3,563,886 and U.K. Pat. No. 1,122,522 disclose regular pore size distribution of 0 to 240 .ANG. with 85% of total pore volume in 50-200 .ANG. range, and suggest that catalysts containing mostly micropores will be poisoned rapidly and asphaltene which penetrates the larger pores subsequently will block entrance to the smaller pores. NPA Pat. No. 6,815,284 discloses the desirability of intermediate pores (100 to 1000 .ANG.) plus channels (&gt;1000 .ANG.) to take up preferentially absorbed large molecules without causing blockage, so that the smaller size pores can desulfurize smaller molecules. German Pat. No. 1,770,996 specifies 0.3 cc/mg of pore volume in diameters larger than 75A and many pores from 1000 to 50,000A, and prefers the open structure for collection of coke and metals.
Commercial catalyst, such as American Cyanamid HDS 1442A, is an effective coal liquefaction catalyst or a petroleum asphaltene hydrodesulfurization and hydroconversion catalyst for a single-stage process. Such single-stage processes are known respectively as H-Coal process and H-Oil process developed by Hydrocarbon Research, Inc. This HDS 1442A catalyst is a special Co-Mo catalyst characterized by its bimodal pore size distribution, with the micropore diameter peaking around 50A and occupying about 2/3 of its total pore volume of 0.7 cc/gm, and the macropore diameter peaking around 2000 .ANG. and occupying about 1/3 of the total pore volume. This catalyst has to serve many fucntions. For example, for coal conversion, the catalyst breaks down the large coal molecules to preasphaltene, and converts preasphaltene to asphaltene, then to distillate liquids and desulfurizes these fractions.
A number of significant advantages are obtained by use of a bimodal catalyst with a suitable surface area of 100 to 250 m.sup.2 /g, as disclosed in U.S. Pat. No. 4,294,685. The catalyst support may be formed of gamma alumina with small pores. The preferred alumina support disclosed in U.S. Pat. No. 3,635,814 is eta alumina. The large pores can be formed by known techniques, such as grinding the alumina to a fine powder and then binding the particles together to form extrudates. During that process, the large pores are generated. This technique for forming large pores is described in U.S. Pat. No. 3,530,066. As disclosed in the art, other pore growth promoting conditions may be used, such as heating the alumina support material in the presence of a gas or a metal compound, steaming at elevated temperatures, etc. In another method, the large pores may be introduced during preparation of the base material by the use of strong mineral or organic acids. Another method involves the addition of a boric acid-phosphoric acid solution to the alumina gel. Still another method is to introduce a relatively large amount of removable materials which may be volatile or decomposable into gases by the application of heat. For example, ammonium carbonate, volatile aromatics, etc., have been employed as removable materials.
Significant process advantages are obtained by using a catalyst with bimodal pore size distribution for coal conversion or for petroleum asphaltene hydrodesulfurization and conversion. For example, such bimodal catalysts are more active and/or deactivate slower. However, the bimodal catalyst still has problems. For coal conversion, high temperature and high hydrogen pressure are needed for achieving high coal conversion, and the preasphaltene conversion capability of this catalyst falls rapidly in the beginning of operations. Analysis of spent catalyst shows that the micropores are filled with carbon deposition within a few days operations, but the macropores contain very little carbon deposition. This indicates that carbon deposition causes pore mouth plugging, thereby resulting in rapid catalyst deactivation, and that catalysts containing micropores smaller than about 50A should be avoided. Metal deposition, such as titanium, causes a further gradual deactivation of the catalyst. For petroleum hydrodesulfurization and asphaltene conversion, carbon deposition and nickel and vanadium deposition cause rapid catalyst deactivation.
Furthermore, a single-stage catalytic hydrocarbon conversion process using a catalyst with bimodal pore size distribution has a main drawback that all the conversions are effected at one reaction temperature, which is the high temperature of about 850.degree. F. needed for rapid conversion of coal or petroleum asphaltene. This high reaction temperature produces undesired high gas yield and high carbon deposition on the catalyst. High gas yield results in a less desirable product distribution and high hydrogen consumption. High carbon deposition on the catalyst causes rapid catalyst deactivation. Thus, there is a need to improve the single-stage catalytic process for hydroconversion of coal and petroleum asphaltene.